1. Field of the Invention
The present invention relates to a technique of determining whether the signal format of each input video signal is an RGB signal or a Y/color difference signal.
2. Description of the Related Art
Direct-view video display devices (also called ‘image display devices) like liquid-crystal displays (LCD) and plasma displays (PDP), as well as projection video display devices (projectors) including liquid crystal display (LCD) panels (may simply be referred to as ‘liquid crystal panels’) or digital micro-mirror devices (referred to as ‘DMD’, trademark by Texas Instruments, Inc.) are typically capable of displaying images expressed by various video signal systems.
The video signals are classified by the picture source, for example, signals output from computers (hereafter referred to as ‘computer picture signals’) and signals used for conventional televisions and hi-visions (hereafter referred to as video picture signals). The video signals are also classified by the signal format, for example, an RGB signal consisting of three primary color signal components R (red), G (green), and B (blue) and a Y/color difference signal (hereafter may simply be referred to as ‘color difference signal’) consisting of a Y (luminance) signal component and two color difference signal components Cr(R-Y) and Cb(B-Y). The video signals are further classified by the resolution of the image expressed by the video signal. For example, the computer picture signals has VGA, SVGA, XGA, and SXGA resolution modes, while the video picture signals has 525i, 525p, 625i, 750p, and 1125i resolution modes.
Adjustment of the processing conditions according to the type of the input video signal is thus required to display images expressed by various types of video signals by one video display device. The video display device accordingly has a function of automatically identifying the type of the input video signal.
A typical video signal identification method detects information on synchronizing signals having different settings corresponding to different types of video signals, for example, settings of a horizontal frequency and a vertical frequency of synchronizing signals representing the timings of the video signal and a number of vertical lines representing the number of horizontal synchronizing signals (hereafter referred to as the ‘synchronizing signal-relating information’), and identifies the type of the video signal, based on the detected synchronizing signal-relating information (hereafter referred to as the ‘synchronizing signal-based identification method’). This synchronizing signal-based identification method is disclosed, for example, in Japanese Patent Laid-Open Gazettes No. 7-87457 and No. 10-312182.
The synchronizing signal-based identification method, however, has difficulties in accurate discrimination between specific types of video signals. For example, the video signal in the VGA mode as one of the computer picture signals and the video signal in the 525p mode as one of the video picture signals have the identical number of vertical lines and substantially similar settings of the frequency of the horizontal synchronizing signal (horizontal frequency) and the frequency of the vertical synchronizing signal (vertical frequency). Accurate discrimination between these video signals is relatively difficult by taking into account the measurement accuracy of the horizontal frequency and the vertical frequency.
A known technique of discrimination between the video signal in the VGA mode and the video signal in the 525p mode utilizes the fact that the video signal in the VGA mode is an RGB signal and that the video signal in the 525p mode is a Y/color difference signal and determines whether the video signal is the Y/color difference signal or the RGB signal (hereafter referred to as the ‘method of RGB/color difference signal discrimination’). The method of RGB/color difference signal discrimination is disclosed, for example, in Japanese patent Laid-Open Gazettes No. 11-155149 and No. 2002-320243.
The prior art method of RGB/color difference signal discrimination utilizes the fact that the Y/color difference signal has a signal level lower than the clamp level but the RGB signal does not have any signal level lower than the clamp level, when the video signal is clamped at the pedestal level. This the prior art RGB/color difference signal discrimination method accordingly compares each video signal with a preset value that is a little lower than the clamp level and determines that the video signal is the Y/color difference signal in the presence of the signal level lower than the preset value.
The prior art method of RGB/color difference signal discrimination may, however, cause wrong discrimination, for example, due to the presence of noise superposed on the video signal. The lower level of the video signal generally leads to the higher potential for wrong discrimination.